Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1    \     2    /   3

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

Solution:

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<int> postorderTraversal(TreeNode* root) {        vector<int> res;        if(root == NULL) return res;        stack<TreeNode*> st;        st.push(root);        TreeNode* flag = root;        while(!st.empty())        {            TreeNode* top = st.top();            if((top->left == NULL && top->right == NULL) ||               (flag != NULL && (flag == top->left || flag == top->right)))            {                res.push_back(top->val);                st.pop();                flag = top;            }            else            {                if(top->right != NULL) st.push(top->right);                if(top->left != NULL) st.push(top->left);            }        }        return res;    }};